The Whammer fingered his necktie knot. As he scooped up the cards his diamond ring glinted in the sunlight.
"Goddamned millionaire," Sam thought. (1.130-31)
The Whammer really hasn't done anything objectionable in the novel. Sam just instinctively hates him because he's got what Sam wants but doesn't have yet: money. The necktie and the diamond ring are signs that the Whammer has made it; he's a big-shot fancy-pants baseball player, and that bugs Sam and Roy because they're nobodies.
"But Walter—he is a successful professional player, isn't he?"
"The Whammer?" Roy nodded.
"And he has won that award three times—what was it?"
"The Most Valuable Player." He had a panicky feeling he was losing her to the Whammer. (1.250-53)
Roy's always conscious of who has the attention of the girls he's interested in. He feels jealous about the Whammer's fame, but that jealousy skyrockets when Harriet notices the Whammer's success.
[Roy's] throwing was quick, strong, and bull's eye.
When Bump got around to his turn in the cage, though he did not as a rule exert himself in practice, he now whammed five of Fowler's fast pitches into the stands. (2.50-51)
Roy isn't the only one who feels jealousy in the novel. Bump, too, starts to notice that his spot as the team big shot is being challenged by the new arrival. He doesn't like it one bit. However, Bump turns his jealousy into positive action, playing better and inspiring his teammates to do so, too.
She walked out of the lobby, with her silver bracelets tinkling, swaying a little on her high heels, as if she had not too long ago learned to walk on them, and went with her beautiful body away, for which Roy everlastingly fried Bump Baily in the deep fat of his abomination. (3.87)
The fact that Bump is Memo's boyfriend drives Roy nuts. The image of Memo, with her jewelry and her sexy walk, is contrasted with the downright hatred that it inspires in Roy. Instead of making him happy to see her, it only makes him curse Bump. It's great how this sentence plunges from words like "tinkling," swaying," and "beautiful" all the way down to a hilariously violent image.
Though Roy denied wishing Bump's fate on him or having been in any way involved in it, he continued to be unwillingly concerned with him even after his death. He was conscious that he was filling Bump's shoes, [. . .] because the crowds made no attempt to separate his identity from Bump's. (4.16)
Nothing like getting rid of your enemy only to have everyone confuse you with him. Poor Roy just can't shake Bump. He gets what he wished for: Memo is all his now that Bump's out of the way. The problem is that Roy's obsession with Bump seems to be stronger than his feelings for Memo, so he's still bugged by Bump even after he dies.
Taking her arm he said, "Memo, I don't know what more I can do to show you how sorry I am about that time and tell you how I feel in my heart for you now."
But Memo stared at him through a veil of tears and said, "I am strictly a dead man's girl." (4.25-26)
Memo isn't giving up on Bump anytime soon, which makes it hard for Roy to shake his jealousy. Her declaration that she is a dead man's girl just lays the cards on the table, and shows that Roy, too, in a weird way, is a dead man's guy. He's in competition with someone who's no longer in the race. How can you win?
Her chair was close to Gus's. Once he chucked her under the chin and she giggled. It sickened Roy because it didn't make sense. (4.155)
Roy is the ultimate denier of his feelings. He can't recognize that he just feels jealous. He has to hide it under other layers. He thinks he feels sick when he watches Memo and Gus flirt because it "doesn't make sense." In a way, it's true, because Gus is a lot older and Memo is out of his league. But really, Roy wants Memo, and he doesn't want anyone else to have her. And that kind of jealousy does make sense in the novel.
Memo said, "Bump was coming up for a Day just before he died."
He felt anger rise in his heart and asked coldly, "Well, Memo, what did he have that I haven't got?" He stood to his full height, strong and handsome. (5.16-17)
In this exchange, Roy is on Cloud Nine. He's just had a day in his honor at the baseball field and received tons of gifts, and he even convinced Memo to take a ride with him in his new car. So when she brings up Bump it brings up all the jealousy he's trying to fight. His little display, peacocking and comparing himself to Bump, is almost a mating ritual. The testosterone's almost visible.
But Bump was dead, he thought, dead and buried in his new box, an inescapable six feet under, so he subtly changed the subject to Gus. (5.20)
Nietzsche said that anybody who lives for combating their enemy has an interest in that enemy staying alive, and Bump is a great example. His jealousy of Bump is one of his primary motivators, so when he loses his great enemy he's got to turn his jealous energies elsewhere. Luckily, Gus is flirting with Memo so he's a natural replacement.
The thought of them sitting peacefully together playing cards gave him the uneasy feeling they might even be married or something. But that couldn't be because it didn't make sense. In the first place why would she marry a freak like Gus? (8.30)
This particular jealous fantasy tells us a lot about how Roy thinks about marriage and domestic life. He doesn't have anyone that he can sit peacefully and play cards with, and it seems like he desperately wants that peace. He saves himself from despair by telling himself that Gus is a freak that Memo would never marry.